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The Metabolic Mill

The breakdown products of the three main classes of foods, carbohydrates, proteins and fats, can participate in glycolysis and the Krebs cycle. These product exist in ‘metabolic pools’ in the cytoplasm, and can be utilized for cell respiration. Thus interconversion of proteins, carbohydrates and fats can occur through the glycolytic pathway and the Krebs cycle. This metabolic interconversion is called the metabolic mill.

(1) Carbohydrates (polysaccharides) are hydrolyzed during the process of digestion to form monosaccharides. Of these the hexose sugar glucose is the key compound in respiration. Other sugars like galactose and fructose can also serve as sources of energy, and are readily convertible into glucose. Glycogen can also be converted into glucose. Glucose is metabolized through the glycolytic pathway and Krebs cycle to yield energy in the form of ATP.

(2) Proteins are digested into their constituent amino acids. These amino acids undergo deamination and enter at various points in the glycolytic pathway and the Krebs cycle. During deamination the amino (- NH2) group of the amino acid is removed. Thus alanine on deamination forms oxaloacetic acid, and glutamic acid forms α-ketogluatric acid.
The amino acid alanine, serine, cysteine, cystine, glycine, theronine, hydroxyproline, valine and methionine enter the glycolytic pathway at pyruvic acid stage.
Phenlaline, tyrosine, tryptophan, leucine and isoleucine enter as acetoacetyl CoA.


The pathway can be reversed under certain conditions. Thus by addition of an amino (-NH2) group (amination) α-ketogluatric acid from glutamic acid. Such amino acid which are synthesized by the cell are called non-essential amino acids.

3) Fats on digestion are broken down to glycerol and fatty acids. Glycerol enters the glyctolytic pathway at the triose-phosphate stage.
Fatty acids undergo β-oxidation, by which two-carbon fragments of acetyl CoA are split off at a time from the fatty acid chain. This ultimately results in complete oxidation of the fatty acid.
It will thus be seen that the three basic organic substances of the cell can be manufactured by interconversion through the glycolytic pathway and the Krebs cycle. The cell is therefore not entirely dependent on outside sources for its materials. Under conditions of carbohydrate scarcity the breakdown products of proteins and fats can also be utilized for providing to the cell. When carbohydrates are in excess, fats can be synthesized from them. Thus eating of excess of potatoes (carbohydrates) results in fatness. Conversely, after prolonged fasting a person becomes thin, as hit fat reserves are utilized for cell respiration. The permits a degree of flexibility in the biochemical system of the cell.

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